Bottom Line:
Magnetic materials have found wide application ranging from electronics and memories to medicine.Not only can we tune its transition temperature in a wide range of temperatures around room temperature, but the magnetization can also be tuned from zero to 0.011 A m(2)/kg through an initialization process with two readily accessible knobs (magnetic field and electric current), after which the system retains its magnetic properties semi-permanently until the next initialization process.We construct a theoretical model to illustrate that this tunability originates from an indirect exchange interaction mediated by spin-imbalanced electrons inside the nanocomposite.

Affiliation: 1] NUS Graduate School of Integrative Sciences and Engineering, National University of Singapore,28 Medical Drive, Singapore 117456 [2] Centre for Advanced 2D Materials and Graphene Research Centre, Faculty of Science, National University of Singapore, 6 Science Drive 2, Singapore 117546 [3] Department of Physics, Faculty of Science, National University of Singapore, 2 Science Drive 3, Singapore 117542.

ABSTRACTMagnetic materials have found wide application ranging from electronics and memories to medicine. Essential to these advances is the control of the magnetic order. To date, most room-temperature applications have a fixed magnetic moment whose orientation is manipulated for functionality. Here we demonstrate an iron-oxide and graphene oxide nanocomposite based device that acts as a tunable ferromagnet at room temperature. Not only can we tune its transition temperature in a wide range of temperatures around room temperature, but the magnetization can also be tuned from zero to 0.011 A m(2)/kg through an initialization process with two readily accessible knobs (magnetic field and electric current), after which the system retains its magnetic properties semi-permanently until the next initialization process. We construct a theoretical model to illustrate that this tunability originates from an indirect exchange interaction mediated by spin-imbalanced electrons inside the nanocomposite.

f4: Exchange parameter and Monte Carlo Simulations of a disordered 3D Ising model with exponential decaying RKKY interactions.(a) Plot of the oscillating part of the exchange parameter [see equation (3)] for several distances between nanoparticles. The constant C dividing Josc is the leading factor in equation (3). We plot both the case where no electron’s spin-flips are allowed (dashed curves) and the case where these are allowed (full curves). (b) Magnetization in terms of the temperature for different strengths of the indirect exchange coupling. (c) Comparison between the experimental and theoretical blocking temperature in terms of the strength of the spin-imbalance (the theoretical fitting parameters used were J0 = 2.2 × 106 and ξ = 5 nm). The Monte Carlo results of panels (b) and (c) were obtained for simulations (with 79507 Ising moments) starting from a highly ordered state (see supplementary information) that use Metropolis algorithm. They explore the phase space region in the vicinity of the global energy minimum, and indicate that the system can show long range order if the clusters are initially aligned by an external magnetic field.

Mentions:
From Fig. 2(b) we estimate the sample’s average electronic densities, n±, finding that they are typically small such that first-neighbor interactions are generally ferromagnetic — see supplementary information. Assuming that then we conclude that J(r, n+, n−) is minimal for spin-imbalance zero, growing with increasing spin-imbalance—see Fig. 4(a). This is in contrast with the typical RKKY result where no spin-flips of the electrons are considered. Our analytical result explains how the ferromagnetic coupling increases with spin-imbalance explaining the experimental observation that the magnetization vanishes without the spin-imbalance and increases with larger spin-imbalance.

f4: Exchange parameter and Monte Carlo Simulations of a disordered 3D Ising model with exponential decaying RKKY interactions.(a) Plot of the oscillating part of the exchange parameter [see equation (3)] for several distances between nanoparticles. The constant C dividing Josc is the leading factor in equation (3). We plot both the case where no electron’s spin-flips are allowed (dashed curves) and the case where these are allowed (full curves). (b) Magnetization in terms of the temperature for different strengths of the indirect exchange coupling. (c) Comparison between the experimental and theoretical blocking temperature in terms of the strength of the spin-imbalance (the theoretical fitting parameters used were J0 = 2.2 × 106 and ξ = 5 nm). The Monte Carlo results of panels (b) and (c) were obtained for simulations (with 79507 Ising moments) starting from a highly ordered state (see supplementary information) that use Metropolis algorithm. They explore the phase space region in the vicinity of the global energy minimum, and indicate that the system can show long range order if the clusters are initially aligned by an external magnetic field.

Mentions:
From Fig. 2(b) we estimate the sample’s average electronic densities, n±, finding that they are typically small such that first-neighbor interactions are generally ferromagnetic — see supplementary information. Assuming that then we conclude that J(r, n+, n−) is minimal for spin-imbalance zero, growing with increasing spin-imbalance—see Fig. 4(a). This is in contrast with the typical RKKY result where no spin-flips of the electrons are considered. Our analytical result explains how the ferromagnetic coupling increases with spin-imbalance explaining the experimental observation that the magnetization vanishes without the spin-imbalance and increases with larger spin-imbalance.

Bottom Line:
Magnetic materials have found wide application ranging from electronics and memories to medicine.Not only can we tune its transition temperature in a wide range of temperatures around room temperature, but the magnetization can also be tuned from zero to 0.011 A m(2)/kg through an initialization process with two readily accessible knobs (magnetic field and electric current), after which the system retains its magnetic properties semi-permanently until the next initialization process.We construct a theoretical model to illustrate that this tunability originates from an indirect exchange interaction mediated by spin-imbalanced electrons inside the nanocomposite.

Affiliation:
1] NUS Graduate School of Integrative Sciences and Engineering, National University of Singapore,28 Medical Drive, Singapore 117456 [2] Centre for Advanced 2D Materials and Graphene Research Centre, Faculty of Science, National University of Singapore, 6 Science Drive 2, Singapore 117546 [3] Department of Physics, Faculty of Science, National University of Singapore, 2 Science Drive 3, Singapore 117542.

ABSTRACTMagnetic materials have found wide application ranging from electronics and memories to medicine. Essential to these advances is the control of the magnetic order. To date, most room-temperature applications have a fixed magnetic moment whose orientation is manipulated for functionality. Here we demonstrate an iron-oxide and graphene oxide nanocomposite based device that acts as a tunable ferromagnet at room temperature. Not only can we tune its transition temperature in a wide range of temperatures around room temperature, but the magnetization can also be tuned from zero to 0.011 A m(2)/kg through an initialization process with two readily accessible knobs (magnetic field and electric current), after which the system retains its magnetic properties semi-permanently until the next initialization process. We construct a theoretical model to illustrate that this tunability originates from an indirect exchange interaction mediated by spin-imbalanced electrons inside the nanocomposite.